A laser is a light generator which typically produces coherent, directional, substantially monochromatic, and intense light. Laser light is generated by inputting energy to a specific medium which stores the energy for some length of time. Much of this energy can be extracted in the form of light by sending photons of an appropriate wavelength into the medium. As the photons pass though the medium they can “stimulate” additional photons, typically identical in every way, to exit the medium. The result is that more photons exit the medium than enter. The medium, therefore, is generally referred to as a “gain” medium, and more specifically, a non-linear gain medium. In order to reach large net gains the photons should be passed through the gain medium as many times as possible. This is routinely done by using an optical “cavity” composed of two or more mirrors (or reflecting surfaces). The mirrors are configured in such a fashion as to maintain the movement of the photons to a single recurring path through the gain medium. The mirrors and materials may be located inside an open cavity, if desired. Usually one of the cavity mirrors is less than 100% reflecting causing a small percentage of light to “leak” out every time photons impinge on it. This mirror is frequently referred to as the “output coupler”. The light “leaking” through the output coupler becomes the laser output which is less intense than the light inside the cavity. As an example, assume that the output coupler is the only lossy mirror, the remaining cavity mirrors being 100% reflecting. The intracavity optical power will be greater than the output beam power by the ratio of the output power divided by the output coupler transmittance (1-reflectance). Now assume that the output coupler is 60% reflecting and the output beam power is 1.3 watts. The circulating (intracavity) power will be 1.3/(1−0.6) or 3.25 watts.
There is normally an optimum reflectance for the output coupler in each laser design which produces maximum output power. Referring to FIG. 1, a basic laser includes a pair of reflecting surfaces M1 and M2 parallel with each other, causing photons to “bounce” between the mirrors many times before escaping the optical cavity (defined by mirrors M1 and M2). M2 is less than 100% reflecting which defines it as the output coupler. The output beam power may be measured (Po). Knowing the reflectivity of M2 (X %), one can calculate the power of the beam inside the optical cavity (Pi) in the following manner: Pi=Po/(1−(X/10)).
Nonlinear optics is the study of phenomena that occurs as a consequence of the modification of the optical properties of a material system by the presence of light. Typically, only laser light is sufficiently intense to modify the optical properties of a material system. Nonlinear phenomena are “nonlinear” in the sense that they occur when the response of a material system to an applied optical field depends in a nonlinear manner upon the strength of the optical field. As an example, second harmonic generation (SHG) occurs as a result of the part of the atomic response that depends quadratically on the strength of the applied optical field. Consequently, the intensity of the light generated at the second harmonic frequency (2ω) tends to increase as the square of the intensity of the applied laser light (fundamental, ω) making SHG a nonlinear process.
Under proper conditions, the process of second harmonic generation can be so efficient that most of the power in the incident radiation at frequency w is converted to radiation at the second harmonic frequency 2ω. One common use of SHG is to convert the output of a fixed frequency laser. For example, a Nd:YAG laser typically operates in the near infrared at a wavelength of 1.063 μm. Second harmonic generation is routinely used to convert the wavelength of the radiation to 0.532 μm, in the middle of the visible spectrum.
Various materials can be used to perform SHG on different wavelengths of laser light. Not all materials will perform well over the entire optical spectrum. Two crystals commonly used to perform SHG with Nd:YAG lasers is Potassium Titanyl Phosphate (KTiOPO4), usually referred to as KTP, and Potassium Niobate (LiNbO3). KTP efficiently doubles 1.064 μm wavelength light and LiNbO3 efficiently double 0.946 μm wavelength light which a Nd:YAG laser can also produce.
The efficiency of the SHG process increases with the incident intensity of the fundamental laser beam. Intracavity laser power is usually higher than the output beam power from the laser. These two factors motivate laser designers to place the SHG crystal (the material) inside the laser cavity along with the gain medium. Smith, Theory of Intracavity Optical Second-Harmonic Generation, IEEE Journal of Quantum Electronics, Volume QE-6, Number 4, April 1970, Pages 215-223, describes the basic fundamentals of intracavity optical second harmonic generation and is incorporated by reference herein. Under this condition the laser cavity mirrors are usually chosen to reflect 100% of the fundamental light to keep the intracavity power as high as possible. The SHG crystal coverts the circulating laser light (fundamental) to a wavelength (second harmonic) which is not reflected by the cavity mirrors. In order to send all the generated second harmonic light out in one beam, all the laser cavity mirrors (with the exception of the output coupler) are designed to also reflect 100% of the second harmonic wavelength. The output coupler is designed to reflect 100% of the fundamental light but transmit 100% of the second harmonic light.
A microchip laser exploits the intracavity doubling concept and couples to it large scale manufacturability and ease of use by the consumer. It is usually fabricated from two crystals (gain medium, SHG) attached together and polished such that their outside surfaces are parallel with each other. Appropriate optical coatings are placed on the outside surfaces of the crystals which subsequently defines the laser cavity. Several examples of microchip lasers are disclosed in U.S. Pat. Nos. 5,610,934, 5,574,740, 4,731,787, and 5,889,798, all incorporated by reference herein. The two crystals together generally forming the “cavity” are constructed using known techniques. A diode layer “pump” provides the input energy to the gain medium of the microchip laser. The diode pump is a device that provides energy to the laser cavity.
Referring to FIG. 2, a laser resonator 10 is normally bounded by end mirrors 12 and 14 which define the laser cavity with length L. End mirror 12 is optically coated to be highly reflective at the lasing wavelength (for Nd:YVO4 1064 nm) and highly transmissive at the pump wavelength (for Nd:YVO4 808 nm). End mirror 14 is optically coated to be highly reflective at the lasing wavelength but highly transmissive at the second harmonic wavelength (for Nd:YVO 5320 nm). The pump energy 16 is directed through the end mirror 12 and excites a volume within the active medium 18. The length of the active medium 18 is typically 1 mm to 5 mm, though any size may be used. Suitable active media crystals include but are not limited to Nd:YVO4, Nd:YLF, Nd:YAG, Nd:GdVO4, etc. The length of the active medium is in part determined by the requirement that the majority of the pump excitation energy be absorbed in the laser crystal.
A non-linear crystal 20 is also disposed in the laser resonator 10. The crystal 20 should typically have a length of about 1 mm to about 5 mm, though any size may be used. Suitable crystal materials include but are not limited to: KTP, LBO, BBO, KnbO3, LiNBO3, etc. Frequently these crystals are non-linear in their response. These crystals are fabricated, properly oriented in the laser cavity, and, if necessary heated to the appropriate temperature to be properly phase matched at the laser wavelength in order to provide efficient frequency doubling. Other materials and configurations may be included in the design of the particular laser, such as shown illustrated in U.S. Pat. No. 5,627,849, incorporated by reference herein.
The output beam from any laser will posses characteristics which are less than ideal. As an example, the output power from a continuous wave (CW) laser will fluctuate over time. This fluctuation (noise) may have many origins. Usually the utility of a laser decreases as the noise increases. Referring to FIG. 3, one technique to characterize the amplitude noise of a laser beam is to direct a portion of the laser beam to be measured onto a fast response photodetector (DC to several tens of MHz). The output of the photodetector is displayed on an oscilloscope capable of accurately resolving the time and amplitude fluctuations of the laser beam. A ratio is made of the AC and DC components (the AC value in volts is divided by the DC value in volts). Multiplying this result by 100 gives the percent value of the noise contained in the laser beam. Other variations of this methodology exist. In a low noise signal the AC component will be a very small proportion of the DC value. The measurement of FIG. 3 shows a near maximum noise content over all frequencies resolvable by the measurement system (oscilloscope with photodetector). Rigorous noise characterization of signals may utilize a spectrum analyzer. Spectrum analyzers measure level versus frequency of the input signal.
What is desirable, therefore, is a laser with minimized noise, especially amplitude fluctuations.